As is known, fencing is frequently used to surround areas which are desired to be kept secure. For example, areas in which secret activities are taking place, warehouses, and prisons, frequently have fencing encircling them to either prevent penetration from the outside in or vice versa. Since the secure area may be very large, and the fencing correspondingly long, it is frequently difficult or impossible for personnel to physically monitor the entire length of it. Thus, it is desirable to provide an apparatus for automatically providing an alarm indication at a central location when the fence has been cut or climbed over, and to further provide an indication of the location of the intrusion so that personnel may be dispersed to the appropriate location to apprehend the intruder.
One intrusion detection apparatus of the prior art utilizes an optical fiber carrying a light signal which is strung around the fence. If the fence is cut or broken, or if it is stressed by someone climbing thereover, the optical fiber breaks with the resultant lost of the transmitted light signal. To locate the break, the fence is divided into sectors or quandrants, each with its own optical fiber, receiver, and transmitter, and signals transmitted in each section or quandrant are coded, with received signals being transmitted to a central monitoring station. Thus, the loss of a particular coded signal at the central station indicates a fence break and identifies the sector or quandrant in which the break or signal loss occurs. While this system is effective to locate fence breaks, it is relatively complicated, as it involves division of the perimeter into sectors and associated coding equipment.
Wolf et al U.S. Pat. No. 3,707,709 provides an intrusion system for detecting an intruder who has cut through or is near a fence wherein a transmission line comprised of the fence and a conductor strung parallel to the fence is utilized. Electrical pulses are fed along the transmission line and are reflected back from the other end of the line, which is open circuited. The reflected, received signal is displayed on an oscilloscope, and if an intruder is near a section of the transmission line or cuts the fence, which forms part of the line, the oscilloscope trace changes. One difficulty with the Wolf et al system is that it requires constant monitoring of the oscilloscope, as it does not provide an alarm signal to alert personnel to the occurence of an intrusion.
Comeaux U.S. Pat. No. 4,023,154 and Redfern U.S. Pat. No. 4,144,530 are also directed to fence intrusion detection systems, but also suffer from certain disadvantages.